Refining cellulose fiber for conversion into cellulose derivatives



Patented May 26,1936 v atness" REFINING CELLULOSE FIBER FOR CON- VERSIONINTO CELLULOSE DERIVA- TIVES George A. Richter, Berlin, N. 11., assignorto Brown Company, Berlin, N. H., a corporation of Maine No Drawing.Application July 25, 1934,

Serial No. 736,901

.19 Claim.

This invention relates to the art of refining cellulose fiber or pulpmore particularly for conversion into cellulose derivatives. While notlim ited thereto, itdeals more specifically with a refining processdesigned to put preliberated cellulose fiber, such as sulphite or otherchemical pulp, in such good condition for acetylation that cellulose,acetate preparedtherefrom is unsurpassed in quality by cellulose acetateprepared from high grade cotton or cotton linters, for instance,yielding cellulose acetate solutions substantially free of haze orturbidity, as hereinafter described.

In accordance with the present invention, preliberatedcellulose fiber,such as sulphite or other chemical wood pulp, is put through therefining action of at least two different alkaline liquors. The firstrefining operation is accomplished with a comparatively dilute hotalkaline liquor, as such a liquor does well the work of ridding the pulpof the more easily removable non-alpha cellulose components,particularly when the pulp has undergone preliminary treatment withchlorine water, hypochlorite bleach liquor or other oxidizing or similarreagent that reacts upon residual ligneous matter present in the pulp.The second refining operation is accomplished with a strong caustic sodasolution at suiiiciently elevated temperature to avoid. sen-,- siblemercerization of the pulp fibers, as I have found that a solution ofsuch causticity as to have appreciable mercerizing activity at roomor'lower temperature does well the work or reducing markedly residualnon-alpha cellulose components, especially pentosans, at non-mercerizingtemperatures. It is most important that the pulp be refined or purifiedwhile being kept in a substantially unmercerized condition, sincemercerized fiber does .not respond properly to acetylation, particularlywhen it has been dried preparatory to acetylation and has thus undergonethe marked shrinkage that attends the drying of mercerized fiber.

The refining operations practiced in accordance with my invention mayadvantageously be supplemented by other chemical treatments or thefiber. In addition to the preliminary treatment of the fiber alreadymentioned, the fiber is preferably bleached to whiteness after itsinitial refinement in the dilute, hot alkaline liquor. Indeed, the fibermay to advantage receive up to its secondary refinement the completeprocessing disclosed in Richter and Schur Patent No.

1,833,976 dated December 1, 1931, which processing material, such as redoil, to the dilute, hot caustic soda solution serving as thepulp-refining reagent. As pointed out in that patent, the soap tends toemulsify or disperse certain undesirable residues present in the fiberand to promote the production of a refined fiber exceedingly low in itscontent of resinous, pitchy, gummy or other color-generating residuessoluble in organic solvents. The initial refinement of the fiber maytake place at a stock consistency of, say, 5% to 10%, in various otherdilute, hot alkaline liquors whose alkalinity is equivalent to that of acaustic soda solution of, say, 0.5% to 2% strength. Thus, the hotalkaline liquor maybe one containing calcium hydroxide, sodiumcarbonate, sodium sulphide, or other suitable alkaline chemical.

Starting with ordinary unbleached sulphite wood pulp, which has an alphacellulose content of about 87% and a pentosan content of about 5%, andputting it through the processing of the aforementioned patent, one mayarrive at a refined pulp having an alpha cellulose content of about94.5% and a pentosan content of about 2.5%. Such refined pulp mayundergo the secondary refinement or purification of the presentinvention in bulk or sheet form. In either case, the pulp is broughtinto contact with excess caustic soda solution 01 at least about 10% to14% strength and at suficiently elevated temperature, say, 50 to 60 C.,to avoid mercerization and its undesirable effects on the pulp. The pulpmay be maintained in contact with such solution for a period up to aboutone hour, at the end of which time the refining or purifying reactionhas proceeded substantially to equilibrium. The fiber is'then washedpreferably with water suificient- 1y hot to avoid mercerizing effectssuch as. might result from the cooling of the refining liquor with coldwater. The final product thus prepared has an alphacellulose content ofabout 96% to 99% and a pentosan content of about 1% to 1.4%. It may bedried and subjected to acetylation as customarily to produce withoutdifliculty excellent cellulose acetate, that is, cellulose acetate thatdissolves in the ordinary cellulose-acetate solvents to yield solutionsof the desired freedom from color and haze and hence admirably adaptedfor use in making films, artificial silk, lacquers, etc. These resultsare attributable to the extremely low pentosan content andextraordinarily high alpha cellulose content of the refined wood pulpconstituting the base of the cellulose acetate.

In carrying out the secondary refining operation on bulk pulp, theprocedure may be to mix then be washed free of the remaining liquor with10 hot water, as in a counter washer, with minimum dilution of theliquor. It is preferable to acidify the washed stock as with dilutesulphurous acid solution and to give it a final washing with water,thereby ensuring a complete removal of caus- 15 tic soda. The washedstock may then be dried,

as by sheeting and drying on a pulpdrier; or it may be otherwise driedand then shredded or fiufied i'or acetylation. a

In carrying out the secondary refining opera- 20 tion of the presentinvention on sheeted pulp,

that is, pulrrthat has been sheeted and dried on a pulp drier to formthick sheets generally known as drier sheets, the procedure may be todraw oil the sheet progressively from a reel and feed 25 it through thestrong alkaline liquor with a time of immersion of, say, 5 to 10seconds, in the bath.

The sheet as it leaves the bath is substantially uniformly saturated orsoaked with solution. It

is preferably lightly squeezed so as to remove 30 excess undilutedsolution for reuse and is then reeled. The reeled sheet impregnatedthroughout with strong caustic soda soiution'is kept for, say,

one hour, whereupon its caustic soda content is removed. The atmospherein which the reeled 35 sheet is kept is preferably heated suificientlyso as to keep the solution in the sheet sufiiciently' hot to be withoutmercerizing efiect on the fibers. If the sheet has previously beensqueezed only lightly, it may be passed through press rolls on 40 itsway to a washer, preferably of the counter current type, whereinto hotwash water is preferably delivered and wherein the caustic soda solutionis recovered with minimum dilution. The press rolls may be set to reducethe solution content of the sheet to not more than, say, about 50% to60%, thereby leading to a recovery of more undiluted caustic sodasolution for reuse. The washed sheet is preferably treated with dilutesulphurous acid solution, again washed, and 50 finally dried. The driedproduct is similar in its composition to that had from the treatment ofbulk pulp. It may be shredded or fiufied for acetylation.

An alternative practice of treating sheeted 55 pulp accordant with myinvention may be generally similar to that followed in theviscose-making industry. Thus, a stack or succession of sheets may beimmersed in strong, hot solution of caustic soda in the usual steepingpress and 60 the sheets may be permitted to soak therein for, say,thirty tosixty minutes, whereupon the plunger of the press may beapplied to remove the excess or removable hot solution from the soakedsheets and the pressed sheets removed 65 from the press, suitablywashed, and dried. Such a batch practice does away with the need ofstoring the-sheets while they are impregnated with the causticsoda-solution as hereinbefore described in connection with theprocessing of a 7 continuous sheet. I

I wish to emphasize the fact that for the purpose of the presentinvention, itis necessary to use a caustic soda solution of at leastabout 10% strength for effecting the secondary refining op- 75 eration.The fact is that it is only when the solution is of 12% to 14% orgreater strength that the refined pulp may be acetylated with as goodresults as those realized from high grade cotton or cotton linters. Asalready indicated, this strong solution must be applied to, and removedfrom, the fiber at temperatures sufiiciently elevated to inhibitmercerizing efiects, since otherwise the fiber would be impaired foracetylationin the sense that it would become passive toward the usualacetylating reagents. The

secondary refining operation of the present invention may, if desired,be applied to low grade cotton linters or to rag stocks that have beensemi-purified and reduced to pulp, as such forms of cellulose can alsobe thus greatly improved 15 for acetylation.

While the principles of the present invention are applicable to variouskinds of preliberated cellulose pulps, including kraft and soda woodpulps, they are of especial value when applied to sulphite wood pulp, assuch latter pulp is characterized by its, comparatively low pentosancontent as a result of the particular fiber-liberating process used inits production, and, accordingly, it can be processed in accordance withthe present invention to produce a refined pulp of extremely lowpentosan content more readily than such wood pulps as kraft and "soda,which are of distinctly higher pentosan content to begin with. Thefactis that when the conditions of sulphite cooking or fiber-liberation arespecially chosen, the processing of the resulting sulphite wood pulp inaccordance with the present invention is attended by even better'resultsthan those hereinbefore indicated. Thus, if the bisulphite cookingliquor is made up with a combined S02 content of only about instead ofthe usual 1% or greater combined S02 content, and with a free SO:content of about 5% to 7%, and if cooking of the wood chips is per- 40formed at somewhat lower temperatures than ordinarily, the processing ofthe resulting pulp as hereinbefore described leads to a refined productwhose pentosan content ranges from 0.5% to 1.0% and whose alphacellulose content ranges from 96% to 99%.

While refined cellulose fiber prepared by the process of the presentinvention is intended more especially for acetylation, nevertheless, itis admirably adapted for conversion into a wide variety of cellulosederivatives, such as cellulose nitrate, cellulose xanthate,cuprammonium, cellulose, cellulose ethers, etc., as its extremely lowpentosan content and extraordinarily high alpha cellulose contentbespeak cellulose derivatives and end-products of highly desiredphysical and chemical characteristics.

An important advantage of the process of the present invention is thatit is aneconomical one in that .the refinement of preliberated cellulosefiber is first carried as far as is practicably pos-, sible in acomparatively inexpensive or dilute alkaline liquor so that the moredifficult subsequent or last-stage refinement can be performed in themore expensive or concentrated alkaline liquor with lower consumption ofthe alkali of such latter liquor, and, accordingly, with a greaternumber of uses of such liquor before it becomes so contaminated that itmust be discarded or subjected to alkali-recovery ppe'rations.

I claim:

l. A process which comprises first refining preliberated cellulose pulpto higher alpha cellulose content in an alkaline liquor oinon-mercerizing strength, and then refining the resulting 2. A processwhich comprises first refining chemical wood pulp to higher alphacellulose content in an alkaline liquor of non-mercerizing strength,then refining the resulting pulp to still higher alpha cellulose contentin another liquor of mercerizing strength while maintaining ,such liquorsufiiciently hot to avoid the mercerizing activity possessed thereby atroom temperature, and washing the pulp free of such liquor whilepreserving the unmercerized condition of the pulp fibers. I

3. A process which comprises refining cellulose pulp in a liquor ofmercerizing strength and activity at room temperature but maintainedsufficiently hot to avoid'the mercerizing activity possessed thereby atroom temperature, and washing the resulting pulp free of such liquorwith wash water at sufilciently elevated temperature to preserve theunmercerized condition of the pulp fibers.

4. A process which comprises refining cellulose pulp in a caustic sodasolution of at least 10% strength while maintaining such solution at atemperature of at least about 50 C., and washing the resulting pulp freeof such liquor with sufiiciently heated wash water to preserve theunmercerized condition of the pulp fibers.

5. A process which comprises subjecting cellu-' lose fiber containingless than 10% of non-alpha cellulose, impurities to the action of acaustic soda solution of at least about 10% strength undernon-mercerizing, elevated temperature conditions to raise the alphacellulose content of such fiber to upwards of 96%, and washing the fiberfree of such solution with wash water at sufiiciently elevatedtemperature to maintain such solution at non-mercerizing activity.

6. A process which comprises subjecting cellulose fiber containing lessthan 10% of non-alpha cellulose impurities to the action of a causticsoda solution of at least about 10% strength under non-mercerizing,elevated temperature conditions to raise the alpha cellulose content ofsuch fiber to upwards of 96%, washing the fiber free of such solutionwith wash water at sum: ciently elevated temperature to maintain .suchsolution at non-mercerizing activity, and acetylating the unmercerizedpurified fiber.

'7. A process which comprises immersing a sheet of purified wood pulphaving an alpha cellulose content upwards of 90% in a caustic sodasolution of at least about 10% strength under non-mercerizing, elevatedtemper ture conditions, keeping the solution absorbed threby in contacttherewith for at least about one hour under such nonmercerizingtemperature conditions, and washing the sheet free of the solution withwash water at sufficiently elevated temperature to maintain the solutionat non-mercerizing activity 8. A process which comprises immersing asheet of purified wood pulp having an alpha cellulose content upwards of90% in a caustic soda solution of at least about 10% strength undernonmercerizing, elevated temperature conditions,

keeping the solution absorbed thereby in contact therewith for at leastabout one hour under such non-mercerizing temperature conditions,washing the sheet free of the solution with wash water at suficientlyelevated temperature to maintain the solution at non-mercerizingactivity, and

acetylating the unmercerized purifled pulp con- 3 stitutingsuch sheet.

9. Substantially unmercerized cellulose fiber substantially free fromingredientssoluble in a caustic soda solution of at least 10% strengthat a temperature of-at least 50 0., said fiber having an alpha cellulosecontent of about 96% to 99%.

and a pentosan content not exceeding 1.5%.

10. Substantially unmercerized wood pulp substantially free fromingredients soluble in a caustic soda solution of at least 10% strengthat a temperature of at least 50 C., said wood pulp having an alphacellulose content of about 96% to 99% and a pentosan content notexceeding 1.5%.

11. A process which comprises first refining preliberated cellulose pulpto higher alpha cellulose content in an alkalineliquor ofnon-mercerizing strength, then refining the resulting pulp to stillhigher alpha cellulose content in an excess of another liquor ofmercerizing strength while maintaining such liquor sufiiciently hot toavoid the mercerizing activity possessed thereby at room temperature,removing a portion of the hot liquor from the pulp, and washing the pulpfree of remaining liquor while preserving the unmercerized condition ofthe pulp fibers.

12. A process which comprises first refining chemical wood pulp tohigher alpha cellulose content in an alkaline liquor of non-mercerizingstrength, then refining the resulting pulp to still higher alphacellulose content in an excess of another liquor whose alkalinitycorresponds to a caustic soda solution of at least about 10% strengthwhile maintaining such liquor. sulficiently hot to avoid the mercerizingactivity possessed thereby at room temperature, removing a portion ofthe hot liquor from the pulp, and washing the pulp free of remainingliquor while preserving the unmercerized condition of the pulp fibers.

13. A process which comprises refining preliberated cellulose pulpas apulp suspension in an alkaline liquor of mercerizing strength andactivity at room temperature but maintained sufiiciently hot to avoidthe mercerizing activity possessed thereby at room temperature, removinga portion of the hot liquor from the pulp, and washing the remainingliquor from the pulp with wash water at slifliciently elevatedtemperature to preserve the unmercerized condition of the pulp fibers.

14. -A process which comprises refining chemical wood pulp as a pulpsuspension in an alkaline liquor whose alkalinity corresponds to acaustic soda solution of at least about 10% strength while maintainingsuch liquor sufliciently hot to avoid the mercerizing activity possessedthereby at room temperature, removing a portion of such hot liquor fromthe pulp, and washing the remaining liquor from the pulp with wash Waterat sufiiciently elevated temperature to preserve the unmercerizedcondition of the pulp fibers.

15. A process which comprises refining preliberated cellulose pulp insheet form in an alkaline soaking liquor of m'ercerizing strength andactivity at room temperature but maintained sufiiciently hot to avoidthe mercerizing activity possessed thereby at room temperature,squeezing a portion of the hot liquor from the soaked pulp cerizingactivity. I

16. A process which comprises refining chemical wood pulp in sheet formin an alkaline soaking liquor whose alkalinity corresponds to a causticsoda solution of at least about 10% strength while maintaining suchliquor suiliciently hot to avoid the mercerizing activity possessedthereby at room temperature, squeezing a portion of the hot liquor fromthe soaked pulp sheet, and washing the sheet free of the remainingliquor with wash water at sufflciently elevated temperature to maintainsuch liquor at non-mercerizing activity.

1'7. Substantially unmercerized, preliberated and refined cellulosefiber, said fiber having an alpha cellulose content of about 96% to 99%,a pentosan content not exceeding 1.5%, and capable of undergoingconversion into a cellulose acetate solution substantially free 0! hazeor turbidity.

18. A drier sheet of substantially unmercerized,

refined cellulose fiber having an alpha cellulose content oi! about 96%to 99%, a pentosan content 5 not exceeding 1.5%, and capable ofundergoing conversion into a cellulose acetate solution substantiallytree of haze or turbidity.

19. A drier sheet oi. substantially unmercerized, refined wood pulphaving an alpha cellulose content of about 96 to 99%, a pentosan contentnot exceeding 1.5%, and capable of undergoing conversion into acellulose acetate solution substantially tree of haze or turbidity.

- GEORGE A. RICHTER.

